Approximately 20% of the populations of the industrialized world become hypersensitive (allergic) upon exposure to antigens from a variety of environmental sources. Those antigens that induce immediate and/or delayed types of hypersensitivity are known as allergens (Breiteneder et al. 1997). These include products of grasses, trees, weeds, animal dander, insects, food, drugs and chemicals. The antibodies involved in atopic allergy belong primarily to the immunoglobulin E isotype (IgE). IgE binds to basophils mast cells and dendritic cells via a specific high affinity receptor FcεRI. Upon exposure to an allergen, allergen-specific IgE antibodies on the cell surface become cross linked leading to the release of inflammatory mediators such as histamine and leukotrienes resulting in physiological manifestations of allergy (Akdis 2006).
Diagnostic tests for allergy involve the detection of IgE antibodies from patients with a specificity to proteins from an allergen source. Typically, an aqueous extract from the allergen source, containing a mixture of proteins, is used in these tests. For most allergen sources, the allergenic proteins present in crude extract have only partly been identified and characterised. Diagnostic test procedures for detection of specific IgE antibodies in patients can either utilize an in vitro immunoassay using serum from the patient, or be a skin prick test (SPT), performed by topical application of the specific extract on the skin of the patient (Wainstein et al. 2007).
In recent years, many important allergenic proteins in the allergenic extracts have been identified and characterized. This has enabled the quantitation of specific IgE antibodies to each of these individual allergenic components, often referred to as component resolved diagnostics (CRD) (Valenta et al. 1999) (Hiller et al. 2002) which in many cases can lead to an improved diagnosis of hypersensitivity (Stumvoll et al. 2003). The use of CRD has also been suggested as an aid in the selection of optimal immunotherapy treatment (Valenta et al. 2007). Further, individual allergens can in some cases be used to enhance the diagnostic sensitivity of an extract by spiking the extract with a component. In conclusion, it is thus of great importance to identify and characterise all important allergenic proteins in each allergen source.
Apart from reducing symptoms of allergy by e. g. anti-histamines, more long-term and curative treatment of allergy can be performed with specific immunotherapy. Application of the disease causing allergenic extract, most commonly either subcutaneously or sublingually, that causes a specific activation of a protective immune response to the allergenic proteins. Although the exact mechanisms are not fully known, such a specific activation of the immune system alleviates the symptoms of allergy upon subsequent environmental exposure of the same allergen (Akdis et al. 2007). A further development of regular immunotherapy has been to use one or several purified allergenic proteins instead of a crude natural extract. Such immunotherapy has been successfully performed for grass pollen allergic patients (Jutel et al. 2005) and it has also been suggested for treating allergy against animal dander (Gronlund et al. 2009).
Horse dander is an increasingly common cause of respiratory allergy (Liccardi et al. 2011), with symptoms including rhinitis, conjunctivitis, bronchial inflammation and asthma. Occupational exposure to horse allergens is a significant risk factor for allergic sensitisation (Tutluoglu et al. 2002) but considerable concentrations of allergens can be detected also in other places such as schools (Kim et al. 2005). IgE sensitisation to horse dander was in one study shown to be associated with a high risk of developing asthma (Ronmark et al. 2003).
Extracts of horse hair and dander contain a complexity of allergenic proteins and four horse allergens have so far been identified: Equ c 1, Equ c 2, Equ c 3 and Equ c 4/5. The first two are both members of the lipocalin protein family and have been purified from their natural source (Dandeu et al. 1993; Goubran Botros et al. 1998) while only Equ c 1 has been expressed as a recombinant protein (Gregoire et al. 1996). The amino acid sequence of Equ c 1 is 67% similar to that of the cat allergen Fel d 4 (Smith et al. 2004). Equ c 3, horse serum albumin, is a relatively conserved protein showing extensive cross-reactivity to other mammalian albumins (Goubran Botros et al. 1996). Equ c 4/5, was first purified and reported as an IgE binding protein in horse dander (Goubran Botros et al. 1998; Goubran Botros et al. 2001) and only later identified as horse sweat latherin (McDonald et al. 2009). Equ c 1 is claimed to be the most important one of the known horse allergens (Dandeu et al. 1993) and IgE antibody recognition of the recombinant protein was present in 76% of a population of horse allergic subjects studied (Saarelainen et al. 2008). In another study using purified native allergens, only 33% of horse allergic patients were sensitized to Equ c 2 and 23% to Equ c 4/5 (Goubran Botros et al. 1998). The frequency of IgE binding to horse serum albumin has been addressed in several studies demonstrating reactivity in up to 40% of horse allergic subjects (Spitzauer et al. 1993; Cabañas et al. 2000). However, as sensitization to serum albumins is often accompanied by higher concentrations of IgE antibodies to other allergen components, its specific clinical relevance is uncertain.
Although the horse dander allergens Equ c 1, Equ c 2, Equ c 3 and Equ c 4/5 have been known for a long time, no quantitative estimation of each component's contribution to the total IgE response to horse dander has been made.